Ethidium bromide is an intercalating dye that was formerly used as a nucleic acid stain in molecular biology. The main three-ring portion resembles a flat purine-pyrimidine base pair, and is able to slip between adjacent co-planar DNA base pairs (intercalation). Under ultraviolet light, intercalated EtBr fluoresces with a strong pink-orange colour, which makes very small amounts of DNA visible. However, ethidium bromide is a very strong mutagen, carcinogen, and teratogen that causes mutations, cancer, and developmental defects, respectively. It has been replaced by other intercalating dyes, such as RedSafe.